I´ve read that they are 245/40 & 265/40 on 18". As you can see, the wider rears have the same aspect ratio as the fronts (40), making those taller overall. This situation is very common on Ferraris, Lamborghinis, etc, but new to BMW (which always had lower aspect ratios for the wider rears).

I do not know the tire sizes for 19". The Geneva concept had Michelin Pilot Cup tires from the E46 CSL in 235/35 & 265/30, but I guess that the production version of the E92 will have even wider tires (maybe 245/35 & 265/35).